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    Subject[PATCH 5.10 446/452] block: fix bio_clone_blkg_association() to associate with proper blkcg_gq
    Date
    From: Jan Kara <jack@suse.cz>

    commit 22b106e5355d6e7a9c3b5cb5ed4ef22ae585ea94 upstream.

    Commit d92c370a16cb ("block: really clone the block cgroup in
    bio_clone_blkg_association") changed bio_clone_blkg_association() to
    just clone bio->bi_blkg reference from source to destination bio. This
    is however wrong if the source and destination bios are against
    different block devices because struct blkcg_gq is different for each
    bdev-blkcg pair. This will result in IOs being accounted (and throttled
    as a result) multiple times against the same device (src bdev) while
    throttling of the other device (dst bdev) is ignored. In case of BFQ the
    inconsistency can even result in crashes in bfq_bic_update_cgroup().
    Fix the problem by looking up correct blkcg_gq for the cloned bio.

    Reported-by: Logan Gunthorpe <logang@deltatee.com>
    Reported-and-tested-by: Donald Buczek <buczek@molgen.mpg.de>
    Fixes: d92c370a16cb ("block: really clone the block cgroup in bio_clone_blkg_association")
    CC: stable@vger.kernel.org
    Reviewed-by: Christoph Hellwig <hch@lst.de>
    Signed-off-by: Jan Kara <jack@suse.cz>
    Link: https://lore.kernel.org/r/20220602081242.7731-1-jack@suse.cz
    Signed-off-by: Jens Axboe <axboe@kernel.dk>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    block/blk-cgroup.c | 8 ++------
    1 file changed, 2 insertions(+), 6 deletions(-)

    --- a/block/blk-cgroup.c
    +++ b/block/blk-cgroup.c
    @@ -1892,12 +1892,8 @@ EXPORT_SYMBOL_GPL(bio_associate_blkg);
    */
    void bio_clone_blkg_association(struct bio *dst, struct bio *src)
    {
    - if (src->bi_blkg) {
    - if (dst->bi_blkg)
    - blkg_put(dst->bi_blkg);
    - blkg_get(src->bi_blkg);
    - dst->bi_blkg = src->bi_blkg;
    - }
    + if (src->bi_blkg)
    + bio_associate_blkg_from_css(dst, &bio_blkcg(src)->css);
    }
    EXPORT_SYMBOL_GPL(bio_clone_blkg_association);


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